FIG. 1 illustrates various types of active implantable and external medical devices 100 that are currently in use. FIG. 1 is a wire formed diagram of a generic human body showing a number of implanted medical devices. 100A is a family of external and implantable hearing devices which can include the group of hearing aids, cochlear implants, piezoelectric sound bridge transducers and the like. 100B includes an entire variety of neurostimulators and brain stimulators. Neurostimulators are used to stimulate the Vagus nerve, for example, to treat epilepsy, obesity and depression. Brain stimulators are similar to a pacemaker-like device and include electrodes implanted deep into the brain for sensing the onset of a seizure and also providing electrical stimulation to brain tissue to prevent the seizure from actually happening. The lead wires that come from a deep brain stimulator are often placed using real time imaging. Most commonly such lead wires are placed during real time MRI. 100C shows a cardiac pacemaker, which is well-known in the art, may have endocardial or epicardial leads. Implantable pacemakers may also be leadless. The family of cardiac pacemakers 100C includes the cardiac resynchronization therapy devices (CRT-D pacemakers) and leadless pacemakers. CRT-D pacemakers are unique in that, they pace both the right and left sides of the heart. The family also includes all types of implantable loop recorders or biologic monitors, such as cardiac monitors. 100D includes the family of left ventricular assist devices (LVAD's) and artificial hearts. 100E includes an entire family of drug pumps which can be used for dispensing of insulin, chemotherapy drugs, pain medications and the like. Insulin pumps are evolving from passive devices to ones that have sensors and closed loop systems. That is, real time monitoring of blood sugar levels will occur. These devices tend to be more sensitive to EMI than passive pumps that have no sense circuitry or externally implanted lead wires. 100F includes a variety of external or implantable bone growth stimulators for rapid healing of fractures. 100G includes urinary incontinence devices. 100H includes the family of pain relief spinal cord stimulators and anti-tremor stimulators. 100H also includes an entire family of other types of neurostimulators used to block pain. 1001 includes a family of implantable cardioverter defibrillator (ICD) devices and also includes the family of congestive heart failure devices (CHF). This is also known in the art as cardio resynchronization therapy devices, otherwise known as CRT devices. 100J illustrates an externally worn pack. This pack could be an external insulin pump, an external drug pump, an external neurostimulator, a Holter monitor with skin electrodes or even a ventricular assist device power pack. Referring once again to element 100C, the cardiac pacemaker could also be any type of biologic monitoring and/or data recording device. This would include loop recorders or the like. Referring once again to FIG. 1, 1001 is described as an implantable defibrillator. It should be noted that these could be defibrillators with either endocardial or epicardial leads. This also includes a new family of subcutaneous defibrillators. ICDs, as used herein, include subcutaneous defibrillators and also CRT-D devices. CRT devices are cardiac resynchronization therapy devices that could also provide high-voltage defibrillation. In summary, as used herein, the term AIMD includes any device implanted in the human body that has at least one electronic component.
FIG. 2 illustrates a prior art cardiac pacemaker 100C showing a side view. The pacemaker electronics are housed in a hermetically sealed and conductive electromagnetic shield 102 (typically titanium). There is a header block assembly 104 generally made of thermal-setting non-conductive plastic, such as Tecothane®. This header block assembly 104 houses one or more connector assemblies generally in accordance with ISO Standards IS-1, IS-2, or more modern standards, such as IS4 or DF4. These header block connector port assemblies are shown as 106 and 106′. Implantable leadwires 110, 110′ have proximal plugs 108, 108′ and are designed to insert into and mate with these header block connector cavities 106 and 106′, or, in devices that do not have header block assemblies built directly into the pulse generator itself.
As used herein, the term “lead” refers to an implantable lead containing a lead body and one or more internal lead conductors. A “lead conductor” refers to the conductor that is inside of an implanted lead body. The term “leadwire” or “lead wire” refers to wiring that is either inside of the active implantable medical device (AIMD) housing or inside of the AIMD header block assembly or both. Furthermore, as used herein, in general, the terms lead, leadwire and pin are all used interchangeably. Importantly, they are all electrical conductors. This is why, in the broad sense of the term, lead, leadwire or pin can all be used interchangeably since they are all conductors. The term “conductive pathway” can also be used to be synonymous with lead conductor, lead, leadwire or pin or even a circuit trace. As described herein, composite conductive sintered paste filled vias passing through an insulator in nonconductive relation with a ferrule electrically acts the same as leadwire, lead wire, or pin. These sintered paste filled vias may also incorporate co-fired solid leadwires. As used herein, the term paste generally refers to pastes, inks, gels, paints, cermets, and other such metal and/or metal/ceramic sinterable material combinations that can be flowable, injectable, pressed, pulled, pushed or otherwise movable into an orifice or via. Post-sintering, the solvents and binders are baked out and, after sintering, the paste becomes a densified solid with monolithic structure. Additionally, AIMD, as defined herein, includes electronic circuits disposed within the human body that have a primary or secondary battery, or have an alternative energy source, such as energy induced by motion, thermal or chemical effects or through external induction. As used herein, the term “header block” is the biocompatible material that attaches between the AIMD housing and the lead. The term “header block connector assembly” refers to the header block including the connector ports for the leads and the wiring connecting the lead connector ports to the hermetic terminal subassemblies which allow electrical connections to hermetically pass inside the device housing. It is also understood by those skilled in the art that the present invention can be applicable to active implantable medical devices that do not have a header block or header block connector assemblies such as pulse generators.
FIG. 3 illustrates a prior art unipolar feedthrough capacitor 132. A quad polar feedthrough capacitor 132 was previously illustrated in prior art FIG. 2. Referring to FIG. 3, one can see that there's an external metallization 142 and a passageway or feedthrough hole metallization 144. This metallization can be applied by electroplating or by applying a fritted glass, which is then fired. In one embodiment, the fritted glass may comprise a silver or palladium silver glass matrix. In any event, after application of the metallization layers 142 and 144, one can make electrical contact to the feedthrough capacitor either by soldering or thermal-setting conductive adhesives or the like. As shown, the feedthrough capacitor comprises active electrode plates 148 and ground electrode plates 146. The reason the electrode plates 146 are called ground electrode plates and as will be further explained herein, is because the perimeter or outside diameter metallization 142 is configured to be attached to a ferrule and in turn, to the conductive housing of an AIMD, which forms an equipotential surface for energy dissipation (aka ground). Referring once again to FIG. 2, one can see that the housing 116, for an active implantable medical device, is generally metallic (titanium). One can also see that the feedthrough capacitor 132 is attached to a hermetically sealed subassembly 120 of the AIMD, which acts as a equipotential surface (ground).
FIG. 3A is taken generally from section 3A-3A from FIG. 3. Shown in exploded view, are ceramic cover sheets 147, active electrodes 148 that are disposed on ceramic layers 149 and ground electrode plates 146 that are disposed on ceramic layers 149. These are stacked up with cover sheets on the opposite end 147 and then pressed and laminated. It will be appreciated that blank cover sheets 147 can be disposed between the active layers 148 and the ground layers 146 thereby, increasing the dielectric thickness and increasing the voltage rating of the device. The electrode layers 148 and 146 are typically applied by silk-screening or equivalent waterfall processes.
FIG. 4 is a cross-sectional view showing the unipolar capacitor 132 of FIG. 3 mounted to a ferrule 112 of a hermetic seal subassembly 120 for an AIMD. As can be seen, the ground metallization 142 of the feedthrough capacitor 132 is electrically connected 152 to the ferrule 112 of the hermetic seal. The hermetic seal is accomplished generally by gold brazing between an alumina insulator 160. There is an outside diameter gold braze 150 between the insulator and the ferrule 112. There is also a gold braze 162 between leadwires 114, 111 and the inside diameter of an insulator 160 passageway 134 as illustrated. In order for gold braze material 150, 162 to wet to the insulator surfaces 160, there must first be an adhesion layer 153 and then a wetting layer 151, as illustrated. In one embodiment, the adhesion layer can be a sputtered layer of titanium, followed by a sputtered layer of molybdenum or niobium (the wetting layer). In some manufacturing agent operations, the adhesion and wetting layers can be combined into a single layer. Throughout the present invention, sometimes in order to simplify, the adhesion layer 153 and wetting layer 151 are not shown or at least not described. But it will be understood that anywhere that a gold braze is described herein to an insulator 160, that an adhesion/wetting layer is required.
As defined herein, what is referred to as the insulator is generally disposed between or inside a ferrule opening and has either lead conductors or conductive passageways or vias that pass through the hermetic terminal subassembly 120. The ceramic capacitor 130 also uses insulative materials, which are dielectrics. As previously described in FIG. 3A, these dielectric sheets 147,149 are referred to as dielectrics although it is appreciated that they are also insulative. In summary, as used herein, insulators are the insulators that are gold brazed to a ferrule of the AIMD, whereas capacitor dielectric insulators are referred to as dielectric layers.
Referring once again to FIG. 4, one can see that the ferrule 112 of the hermetic seal has been laser welded 154 into the overall housing 116 of the AIMD. This is very important in that the feedthrough capacitor ground metallization 142 becomes part of the overall electromagnetic shield of the AIMD housing. This forms in the industry what is known as a Faraday cage and provides an effective electromagnetic interference shield and energy dissipating surface. Referring back to FIG. 4, lead 114 on the body fluid side is generally connected to implanted leadwires and electrodes (not shown). Referring back to FIG. 2 for a prior art pacemaker, one can see these leadwires 107 and 107′ that are connected to electrodes 109 that are located within the human heart. Again, referring to FIG. 2, undesirably, electromagnetic interference (EMI) can be coupled to these implanted leads and in turn, to the interior of the AIMD housing. It has been shown in numerous articles that EMI can disrupt the proper operation of the AIMD, such as a cardiac pacemaker and lead to improper therapy or even complete inhibition of therapy. Inhibition of therapy, for a cardiac pacemaker, can be immediately life-threatening to a pacemaker dependent patient.
Referring once again to FIG. 4, electromagnetic interference signals therefore, may be conducted along leadwire 114 to terminal 1 of the feedthrough capacitor. It is the purpose of the feedthrough capacitor 132 to divert unwanted high-frequency EMI signals from the leadwire 114, 111 so that by the time the signals reach terminal 2 (the AIMD electronics or device side), that the electromagnetic interference has been greatly attenuated or diverted through the feedthrough capacitor, harmlessly to the AIMD housing 116. This is further appreciated by looking at the schematic diagram of FIG. 4A. Electromagnetic interference signals enter terminal 1 of the 3-terminal feedthrough capacitor and are diverted harmlessly to the ground terminal 3 (116) before they can reach the device side 111, terminal 2.
The feedthrough capacitors 132, when properly installed, acts electrically as a continuous part of the titanium shield 116, which houses the active implantable medical device (AIMD). The feedthrough capacitor is a 3-terminal coaxial device whose internal electrode plates “plug the hole” and both reflect and absorb EMI fields. The feedthrough capacitor is novel in that, it is a broadband low pass filter, which allows desirable frequencies (like pacing pulses) to pass. Because it is a unique 3-terminal coaxial device, it provides effective attenuation to undesired signals (EMI) over a very broad band (10 MHz to 10 GHz frequency range). When designed and installed properly, feedthrough capacitors are very low inductance devices, which do not series resonate. It is very important that feedthrough capacitors be installed in such a way that undesirable resistances, for example, due to titanium oxides, cannot occur in the ground connection.
FIG. 5 is very similar to the schematic of FIG. 4A, except in this case, there is an oxide Roxide, as illustrated. This oxide comes from undesirable oxidation of the ferrule 112 previously illustrated in FIG. 4. The electrical connection material 152 illustrated in FIG. 4, is connected to a titanium surface of the ferrule 112. As will be explained, titanium can undesirably form oxides, which become resistive and reduce the effectivity of the feedthrough capacitor 132.
FIG. 6A is taken from FIG. 21 of U.S. Pat. No. 5,333,095, the contents of which are incorporated herein fully by reference. Referring once again to FIG. 6A, one can see that there is a feedthrough capacitor 132 that is mounted onto a ferrule 112 of a hermetic seal subassembly 120. In this case, the diameter of the bipolar feedthrough capacitor (in this case, two passageways instead of one) greatly overhangs the ferrule 112. In this assembly, the steps are first, that the ferrule 112 (without the presence of the feedthrough capacitor 132) is first captured by two AIMD can halves 116, 116′. These are captured as a sandwich and then laser weld is performed between the ferrule 112 and the can halves 116, 116′. The feedthrough capacitor 132 has been subsequently added and an electrical connection has been formed between the feedthrough capacitor ground metallization 142 and to the exposed areas of the AIMD housing 116, 116′. In other words, in this case, there is no direct electrical connection between the feedthrough capacitor ground metallization 142 and the ferrule 112. However, as previously described in FIG. 5, FIG. 6B illustrates the schematic of the bipolar feedthrough capacitor of FIG. 6A. Again, undesirably, an oxide Roxide appears between the bipolar feedthrough capacitor and ground. As will be explained, this can seriously degrade filter performance.
FIG. 7A illustrates a quad polar feedthrough capacitor (meaning four passageways). It will be appreciated that any number of feedthrough holes 134, 144 can be produced. As previously described for the unipolar capacitor of FIG. 3, the quad polar capacitor of FIG. 4, has a ground metallization 142 and four passageways, each having their own active metallization 144. As used herein, the term active means an electrically active lead or passageway as opposed to a grounded connection. Active passageways may conduct therapeutic pacing pulses, biological sensing signals or even high-voltage therapeutic shocks. For a neurostimulator application, active passageways may include AC, pulse, triangular or many other different types of waveforms; for example, for a spinal cord stimulator to create paresthesia.
FIG. 7B is taken generally from FIG. 7B-7B from FIG. 7A, which illustrates the quad polar feedthrough capacitor in cross-section. One can see that there are ground electrode plates 146, which are disposed through the feedthrough capacitor structure and connected to the ground metallization 142. One can also appreciate that each of the feedthrough holes 134 has its own set of active electrodes 148 that are disposed and overlapping or sandwich-type construction between the ground electrode plates 146. It is the overlapping of the active and ground electrode plates in the dielectric that create the individual feedthrough capacitors. Each of the four feedthrough capacitors are associated with its own passageway metallization 144.
FIG. 8 is an exploded view of the unipolar capacitor previously illustrated in FIGS. 7A and 7B. As for the unipolar capacitor of FIG. 3A, there are cover sheets 147 and then an active layer showing four active electrodes 148 that are each individually associated with one of the four passageways. As one can see, the ground electrode layer 146 extends in non-conductive relationship with the active passageways to the feedthrough capacitors outside diameter. As before, these are stacked up in interleave relationship to form a quad polar feedthrough capacitor.
FIG. 9 is the schematic drawing of the feedthrough capacitor of FIG. 8, but in this case, this is after the feedthrough capacitor has been installed to a hermetic seal ferrule and insulator with pins, as previously described in FIGS. 2, 4 and 6A. It is assumed that the feedthrough capacitor outside diameter metallization 142 has been connected directly to either the titanium ferrule 112 or the AIMD housing 116. In both cases, the ferrule and/or the housing would be of titanium and would be subject to oxidation. Accordingly, in the schematic drawing of FIG. 9, one can see that there is an undesirable Roxide shown between each of the feedthrough capacitors 132 and ground 116 (AIMD housing). Referring once again to FIG. 9, one can see that each of the feedthrough capacitors 132 is labeled with terminals 1, 2 and 3. At DC or direct current, there is no difference between terminals 1 and 2 as that is a solid through-pin or leadwire or passageway. However, at RF frequencies, the feedthrough capacitor 132 substantially attenuates frequencies coming from the body fluid side from terminal 1 into the inside of the AIMD housing or device side to terminal 2. As previously stated, these undesirable EMI signals that are entering at terminal 1, are diverted by capacitive reactance through the feedthrough capacitor to ground terminal 3.
FIG. 10A is taken from FIG. 1 of U.S. Pat. No. 5,867,361 also known as the Wolf Patent, the contents of which are incorporated herein fully be reference. Referring to FIG. 10A, one can see that Wolf has a feedthrough capacitor 132 that is connected to a ferrule structure 112. This electrical connection between the ferrule and the capacitor is accomplished by an electrically conductive adhesive 125 as indicated. Then, electrically conductive adhesive 123 connects the capacitor to the device side wire bond pad 121. Wolf contemplates a gold braze area 131, which is the small triangular shape shown in cross-section. Wolf teaches away from a solid through-pin throughout his patent. As one can see, Wolf has a pin 133 disposed on the body fluid side. This pin 133 is discontinuous and is attached by gold braze 127 to insulator structure 160. The insulator is hermetically sealed and mechanically attached to the ferrule by another gold braze 129. In other words, there are three gold brazes described by the Wolf invention. On the device side, there is a wire bond pad 121, which is taught to be discontinuous from lead segment 133. A major deficiency of the Wolf invention is that, the triangle or gold braze area 131 cannot possibly stay in place during the high-temperature gold brazing operation, which typically occurs in a gold braze furnace. It is well known to those skilled in the art that the feedthrough capacitor 132 cannot be present during such high-temperature brazing operations. Not only would the capacitor structure itself be harmed, but its terminations 142 and 144 (not shown in Wolf but described) would be significantly damaged during such high-temperature gold braze operation.
Accordingly, FIG. 10B illustrates the gold brazing operation before the feedthrough capacitor 132 of Wolf is added. As one can see, there is a gold braze preform 131 which is placed (and would generally be a round or O-ring type of cross-section prior to high-temperature application). There are also other gold braze preforms 127 and 129. These are idealistic shapes or even fanciful shapes. As illustrated in FIG. 10C, at high-temperature, once these three gold brazes become molten, they will not stay in place because there is nothing to retain them. In other words, when liquid, gravitational forces will pull 131 generally down into the area as illustrated in FIG. 10C and away from the electrical attachment area 125. A thin layer of gold braze 131 would be left behind, but those skilled in the art realize that thin layers of gold do not provide a significant barrier against oxides. It is commonly known, for example, in the plating of various types of leadwires that first a nickel barrier is laid down and then a final gold layer is laid down. The nickel layer is a barrier layer, such that oxides will not penetrate through it and then the thin layer of gold on top of the nickel provides an oxide-resistant wetting layer for solders and the like.
Referring back to FIG. 10C, one will also see that gold braze 127 cannot possibly retain the shape as shown in FIG. 10A or 10B. Again, once it becomes liquid, due to gravitational forces, it will slump down as indicated in FIG. 10C. In summary, the major deficiency of the Wolf invention to prevent the undesirable oxides Roxide, as illustrated in FIG. 6B and FIG. 9, is that Wolf's gold braze GB3 is never retained and therefore, cannot form a reliable low resistance connection between the feedthrough capacitor 132 ground metallization and the ferrule 112.
FIG. 11 illustrates a prior art rectangular feedthrough capacitor 132, which has the same number of poles (4, quadpolar) as previously illustrated in FIG. 7A. This illustrates that feedthrough capacitors can be round (sometimes called discoidal), rectangular or even any other shape. As previously mentioned, the feedthrough capacitor can be quad polar, as illustrated, or any other number of feedthrough holes 144. Referring once again to FIG. 11, the ground metallization 142 is shown being brought out to both of the long sides of the feedthrough capacitor 132. This is best understood by referring to FIG. 14, which is taken generally from section 14-14 from FIG. 11. This illustrates the ground electrode plates and the fact that they are only exposed along the capacitor's long sides where metallization 142 can be applied. Also shown as FIG. 13, which is taken generally from section 13-13 from FIG. 11, illustrating four active electrodes 148. Each of these active electrodes is associated with one of the active terminal pins 111, 114. The feedthrough capacitor, as illustrated in FIG. 11, is shown ready for installation on top of a hermetic seal subassembly 120 that's illustrated in FIG. 12. Referring to FIG. 12, one can see that there is a metallic ferrule, which is typically of titanium, an insulator, which is typically of alumina and four pins or leadwires 111, 114. A hermetic and mechanical seal is effective between each of the pins 111, 114 and the insulator 160 by gold brazes 162. Also, the rectangular perimeter of the alumina insulator 160 is shown gold brazed 150 to the ferrule 112.
FIG. 15 illustrates the feedthrough capacitor 111 installed to the hermetic seal assembly 120, as previously described in FIGS. 11 and 12. As can be seen, there is an electrical connection material 152, which connects from the capacitor's ground metallization 142 directly to the ferrule 112.
FIG. 16 is taken generally from section 16-16 from FIG. 15. In this section, one can see that there is a gold braze 150 that forms a mechanical and hermetic seal between the insulator 160 and ferrule 112. There is also a hermetic seal gold braze 162 between the insulator 160 and leadwire 111, 114. In this case, the feedthrough capacitor 132 is generally larger in diameter than the gold braze hermetic seal area 150. In this case, one can see the electrical attachment material 152 connecting between the capacitor 132 ground metallization 142 into the ferrule. Layer 164 illustrates a highly undesirable oxide layer on the titanium surface of ferrule 112. Oxide layer 164 would appear all over the surfaces of the titanium ferrule 112 but is shown disposed only between the electrical attachment material 152 and the ferrule for simplicity. Referring once again to FIGS. 15 and 16, one can see that the ferrule 112 has an h-flange type shape 163. This is for capturing and subsequent laser welding of AIMD housing halves 116.
FIGS. 11 through 20 herein were all taken from FIGS. 11 through 20 of U.S. Pat. No. 9,427,596, the contents of which are incorporated herein fully by reference. The '596 patent includes a detailed technical description of the capacitor's equivalent series resistance, the importance of parasitic resistance (ohmic loss) and the problem with oxides of titanium.
FIG. 17 is a schematic diagram illustrating the undesirable presence of Roxide in the ground path of the quad polar feedthrough capacitor. This Roxide results from the oxide layer 164 previously described in FIG. 16.
FIG. 18 shows the use of novel gold braze bond pads 165 that are one embodiment of a novel feature of the '596 patent. This is best understood by referring to FIG. 19 showing that the feedthrough capacitor 132 ground metallization 142 is electrically attached 152 by a thermal-setting conductive adhesive or a solder or the like directly to this gold bond pad area 165. It is well known that gold is a very noble material and does not oxidize. When sufficiently thick, a layer of gold will effectively block titanium oxides from interfering with the high-frequency electrical connection material 152. This is best understood by referring to FIG. 20, which is taken from section 20-20 from FIG. 19. In the cross-section, one can see the electrical connection material 152 that effects a very low impedance and low resistant electrical connection between the feedthrough capacitor ground metallization 142 and the gold braze pad area 165. During gold brazing, the gold braze pad 165 forms a continuous part of the hermetic seal 150 that effects a mechanical and hermetic joint to the insulator 160. In other words, an essential feature of the '596 patent, is that the low impedance, low resistance ground attach area is continuous with and one of the same width, as the same hermetic seal 150 that forms the hermetic seal gold braze.
FIG. 20A is taken from section 20A-20A from FIG. 19. FIG. 20A shows a serious deficiency in the central design concept of the '596 patent. That is, the gold braze 165 is not contained in such a way that it cannot be affected by gravitational forces during high-temperature gold braze furnace operations. As previously mentioned, during high-temperature gold brazing operations, the gold becomes molten or even liquid and due to gravitational forces, can flow out of a defined pocket area as previously illustrated in pocket area 165 in FIG. 20. In other words, it has a tendency to run down, as shown as 165′ in FIG. 28. Again, as with the deficiency in Wolf, this does leave behind a thin layer of gold, however, in order to overcome this, an excessive amount of gold must be used, and one must rely on reduced contact area for the electrical connection material 152′, as illustrated. By carefully controlling the fit-up tolerances between the alumina insulator 160 and the inside diameter of the ferrule 112, one can minimize the gold braze gravitational rundown 165′. However, this makes for a more expensive process and the need to hold the individual pieces to very tight tolerances.
Referring once again to FIG. 20A, one can see that there is a thin layer of gold 165T that is left behind. The problem is that this uncontrolled thin layer may or may not be oxide-resistant. As previously described, when gold becomes too thin, oxides can penetrate right through the relatively porous gold surface. Referring once again to FIGS. 11 through 20 herein, there are also taken from FIGS. 14 through 22 of U.S. Pat. No. 6,765,779, the contents of which are also incorporated herein fully by reference. The '779 patent includes extensive detail on undesirable formation of titanium oxides and a capacitor's equivalent series resistance, including a description of both dielectric loss tangent and ohmic losses.
FIGS. 21 and 22 herein are taken from FIGS. 23 and 24 of the '779 patent. FIG. 21 illustrates that the electrical connection material 152 contacts between, in this case, a round quad polar capacitor's ground metallization 142 and the gold braze area of the hermetic seal 165. This is best understood by referring to section 22-22 from FIG. 21, which is illustrated in FIG. 22. Referring to FIG. 22, one can clearly see that the electrical connection material 152, which can be of thermal-setting conductive adhesive or a solder or the like, makes a low resistance/impedance (free of titanium oxides) connection between the capacitor ground metallization 142 and at least a substantial portion of the gold braze pad area 165, which also forms the hermetic seal between the ferrule 112 and insulator 160. This forms an oxide-resistant low impedance and low resistance electrical connection that would be robust at high-frequencies so that the feedthrough capacitor 132 can properly divert unwanted high-frequency EMI energy. As defined herein, an EMI filter hermetically sealed assembly for an active implantable medical device, will be herein designated as assembly 210. The '779 Patent has enjoyed great commercial success and has proven to be highly reliable. Manufacturing processes of the '779 Patent does require tight dimensional tolerances between the ferrule inside diameter and the alumina insulator outside diameter or perimeter. In addition, the oxide-resistant pads as described in the '779 Patent require a significant amount of extra gold to be used in the process which is thereby increasingly expensive.
FIG. 22A is taken from section 22A-22A from FIG. 21 and illustrates what happens to the gold braze 165 during high-temperature gold braze furnace reflow operations. Due to gravity, since there is nothing to physically constrain the gold, when molten or liquid, it has a tendency to flow down in the area 165′, as illustrated. This leaves behind a much smaller area of gold 165T for electrical attachment 152′ to the feedthrough capacitor ground metallization 142. Referring once again to FIGS. 22 and 22A, a step ST has been added in the ferrule in an attempt to slow down and prevent the flow of the gold braze material 165T. This is somewhat effective but requires that a great deal more gold 165 must be used before this column is filled up, such that a suitable electrical connection 162′ can be accomplished between the feedthrough capacitor ground metallization 142 and the gold braze surface 165T.
FIG. 23 is an electrical schematic taken from FIGS. 20 through 22A wherein the Roxide is no longer present.
FIG. 24 illustrates filter performance otherwise known as attenuation or insertion loss curves vs frequency. An ideal feedthrough capacitor C,132 attenuation curve is shown. One can see that it has a slight self-resonance above 1 GHz and then continues to function. Accordingly, it becomes a broadband 3-terminal filter as previously described. As can be seen, the ideal feedthrough capacitor has over 30 dB of attenuation at all frequencies above 450 MHz. This frequency range is important because that's the range at which cell phones operate. Cell phones are of particular concern to active implantable medical devices because they are small and can be brought into very close proximity to a medical implant. For example, one concern is for a pacemaker patient where the cell phone may be placed in a shirt pocket directly over the implant. This would couple maximum energy to implanted leads. Referring once again to the insertion loss attenuation curves of FIG. 24, one can see what happens when the feedthrough capacitor has undesirable resistive oxide Roxide in its ground electrical path. The oxide degrades the attenuation or filter performance such that you end up with a curve, which provides less than 30 dB of attenuation at frequencies above 450 MHz. This seriously degraded filter performance is of great concern because if a closely held emitter, such as a cellular telephone, interferes with, for example, a pacemaker sense circuit, it can undesirably cause the pacemaker to inhibit. Inhibit means that it would fail to provide life-saving therapeutic pulses. One might ask, why are pacemakers designed to inhibit? Well, there are two reasons: Many patients who suffer from bradycardia (a very low heart rate) are not bradycardic all-day long. In other words, they can come in and out of bradycardic (life-threatening) condition. Therefore, demand pacemakers were developed such that when a patient's normal sinus rhythm returns, the pacemaker will inhibit. This is to not only save battery life, but also prevents a condition called rate competition. This is where you wouldn't want the pacemaker to provide a pulse that is out of sync or competitive with a patient's intrinsic rhythm. However, this does lead to electromagnetic interference danger. If EMI is undesirably detected as a normal cardiac pulse, it can cause the device to inhibit, which is immediately life-threatening for a pacemaker dependent patient.
Accordingly, there is a need to provide an oxide-resistant electrical connection between the filter capacitor ground termination and the hermetic seal ferrule that embodies the present invention, which is a gold pocket pad, and wherein, the gold pocket pad reliably forms a thick enough metallurgically bonded layer to form an oxide-resistant surface to which a filter capacitor ground electrical connection can be made. As carefully described, none of the prior art provides a defined pocket (swimming pool) to capture the gold pocket preform in such a way that it cannot undesirably flow during high-temperature operations such as during hermetic seal gold brazing.